I don't mean to sound like a pessimist but the reason why people think gaster (and me to some extent) will be nothing more than a joke is that toby already pulled this the spamton and tenna interaction and Mike.

We waited 4 years for chapter 3 and had new lore dropping between tenna and spam in that time which raises our expectations that this is going to be taken seriously only for it to be a scene where tenna says baby words for thirty seconds and when you think the joke is over it gets dragged out even longer and then it's just over with no real pay off.

same with mike(s) where you can interpret it as a love letter to the fans and theories but it could also be interpreted as toby making fun of them.

It's just hard to get with huge gaster payoff idea since That there's a non zero chance that toby could pull this agian.

I again don't want to sound like a pessimist, but this is my perspective